An illumination system is an electrical device that can be used to create artificial light or illumination. In the context of this invention a solid state illumination system 1000 generally uses one or more than one light emitting diodes (LEDs) 100 (see FIG. 1) to produce white or coloured light 200.
As is well-known, an LED 100 comprises a chip of semi-conducting material doped with impurities to create a p-n junction. FIG. 2 shows typical current-voltage characteristics for an LED. An LED, when in forward bias 201, emits light 200, and can act as a photosensor when working in reverse bias 202 (Mims, Forrest M., III, LED Circuits and Projects, Howard W. Sams and Co., Inc., New York, N.Y., pp. 60-61, 76-77, 122-123).
FIG. 3 shows the photocurrent 2 generated by an LED as a function of the bias voltage across the LED, measured for a phosphor converted WLED at three different light levels. It can be seen that the photocurrent varies significantly with the level of ambient light when the LED is driven at or near zero bias 3, and this makes it possible to use an LED as a photosensor, for example to sense the level of ambient light. FIG. 4 shows normalised photo response curves for red, green and blue LEDs (driven at zero bias in each case), and these curves indicate that these LEDs can individually detect specific spectral areas of the ambient light. For example, the red-emitting LED can detect ambient light in the wavelength range of approximately 515 nm to 680 nm, with half-maximum sensitivity occurring over the wavelength range of approximately 580 nm to 640 nm.
An LED generally produces light in one or more defined wavelength ranges. Different approaches have been adopted to produce light of a particular desired colour balance (e.g. using multicolor LEDs or phosphor converted LEDs) and these have been described in details elsewhere (for example, by A. Zukauskas et. al., Introduction to Solid State Lighting, A Wiley-Interscience Publication, 2002).
Phosphors are fluorescent materials which absorb primary light of certain energy from a suitable source, referred to as a pump light source, and emit light at a different energy. The secondary emission achieved from these phosphors can be of higher (up conversion) or lower photon energies (down conversion) compared to that of the primary light from the pump light source. The total light output from such devices can purely be secondary emission from phosphors or a mixture of primary light emission from the pump source and secondary light emission from the phosphors. Phosphors are usually made from a suitable host material such as oxides, sulphides, selenides, halides or silicates of zinc, cadmium, manganese, aluminium, silicon, or various rare earth metals, to which an activator is added. The best known type is a copper-activated zinc sulphide and the silver-activated zinc sulphide (zinc sulphide silver).
Recently, nanophosphors have been used in place of, or together with, conventional phosphors. Nanophosphor particles are relatively small in size, typically less than 50 nm as compared to the tens of microns for conventional phosphors. A multitude of colloidal quantum dots is an example of nanophosphor.
Although SSL is energy efficient, smart and intelligent SSL systems are sought for dynamic performance, automated control and further energy savings. For example, where an SSL is used as a backlight for a display it may be desirable to control the intensity of the backlight in dependence on the level of ambient light. Conventionally, a photosensor 4 has been used with a driver 1, to regulate by feedback 300, the luminaire's 1001 output level according to the ambient light level 400 as detected by the photosensor 4 which acts as an ambient light sensor (see FIG. 5).
The above mentioned driver is typically designed to drive the LEDs in Pulse Width Modulation (PWM) mode. In this case the LED(s) of the luminaire 1001 can be switched between forward 201 and reverse bias 202 by applying suitable voltage pulses. Fine-tuning to the output illumination characteristics of the LEDs can be carried out by adjusting the driver forward voltage and pulse width.
A block diagram of the most general arrangement capable of operating a single LED in the two modes of “drive” and “detection” is shown in FIG. 6. To operate in detection mode (switch 12 open and switch 11 closed) the requirement is that a negative, zero or small positive bias V=V1 is maintained across the terminals of the LED 100 by detection circuitry 13, and the current I generated by the LED 100, is measured. For a suitable choice of V1 this current I will be sensitive to the level of illumination incident upon the LED (as shown in FIG. 3). The range of suitable values of V1 will depend on the characteristics of the LED 100 used. In general V1 is best chosen to be a value at or close to the “built in voltage” of the LED, Vbi, which is defined as the bias voltage for which zero current flows when the LED has no ambient light incident upon it. The built in voltage corresponds to the most sensitive region for operating the LED in the detection mode. Operation in the most sensitive region of the device (close to the built in voltage) may be aided by series-connecting multiple devices as described in co-pending UK patent application No. 0619581.2. Note also that for operation in detection mode the LED will not emit significant amounts of light.
In the emission mode of operation (switch 11 open and switch 12 closed) a positive bias V=V2 is maintained across the terminals of the LED. There is no requirement to measure the current that flows through the LED. The intensity of light emitted by the LED is dependent on the value of V2, which is controlled by driver circuitry 16. Thus it is possible to vary the brightness of the LED by varying V2. An alternative method of setting the brightness of the LED is to pulse the bias voltage across the LED between a small bias V0 at which there is no significant light emission and a value of V2. The perceived brightness of the LED can then be adjusted by varying the fraction of time for which the bias voltage pulse is at the high level V2.
An example of a practical implementation of the circuits of FIG. 6 is shown in FIG. 7. This circuit comprises the LED 100, a pulsed voltage source 17 for providing voltage pulses of pulse high level V2, a measuring circuit and an analogue-to-digital converter (ADC) 18, an operational amplifier 19, a DC voltage source V1 20, capacitor 21 and a reset switch 22.
To operate the circuit of FIG. 7 in detection mode switch 11 is opened and switch 12 is closed.
The amount of light incident upon the LED is then measured as follows.                At the start of the integration period, the signal 23 turns on (closes) switch 22 and the output of the integrator is reset to V=V1.        The integration period begins when the signal 23 turns off (opens) switch 22. The photocurrent is integrated on the integration capacitor 21.        The ADC 18 has its input 24 connected to the integration capacitor 21 and may be used to convert the integrated voltage to a digital value at the output 25 of the ADC. It may be configured in either sample-and-hold or continuous time operation. In sample-and-hold configuration, the output voltage from the integration capacitor 21 is sampled once at the end of the integration period and converted to a digital value. In continuous time operation, the ADC is configured to compare the integrated voltage to a fixed reference. The time taken for the integrated voltage to reach this reference is measured by a digital counter which provides the ADC output value at the end of the integration period.        
To operate the circuit of FIG. 7 in the drive mode, switch 12 is opened and switch 11 is closed. Under these conditions the pulsed voltage source 17, which is of a standard type is used to apply a pulsed bias voltage across the LED 100. The amplitude of the pulse high voltage level V2 and the duty cycle of the pulse are chosen as per the requirement for the amount of light emitted by the LED.
The digital output value of the ADC 18 that is representative of the incident ambient light level may be processed in several ways according to the requirements of the overall system. This could be done for example with a simple digital processor system having the ability to perform simple digital operations such as comparing digital signals, storing digital signals in memory and performing simple arithmetic operations. Examples of circuits for performing such operations are very well known and can be found for example in basic level electronics textbooks (e.g. Digital Fundamentals (7th Edition) Floyd Electronics Fundamentals Series, Thomas L. Floyd).
Various lighting systems that allow the output intensity to be varied are known. U.S. Pat. No. 6,469,457 describes a lighting system comprising a plurality of light fixtures, each of the said light fixtures have external dimmers which are capable of adjusting the intensity of the light fixtures. U.S. Pat. No. 6,900,735 describes an illumination device that responds to a multitude of stimuli and adapts to a multitude of purposes, such as measuring ambient temperature, using a sensor. U.S. Pat. No. 6,963,175 describes a light emitting diode illumination control which can comprise temperature sensors, displacement detectors etc. to adjust the illumination system. Agilent's HSDL-9000 ambient light sensors (peak responsitivity at 510 nm) are suitable for regulating LCD backlights and prolong battery life.
U.S. Pat. No. 6,552,495 describes a control system comprising an external sensor designed for generating a desired colour from the light source using a plurality of Red, Green and Blue light emitting diodes (LEDs).
Avago Technologies illumination colour management (ICM) solution (described in U.S. Pat. No. 7,212,287) consists of a colour controller and a colour sensor to enable an RGB (red, blue and green) LEDs closed loop feedback system. ICM is a closed-loop feedback system, which monitors the hue (colour) and intensity (brightness) of the LEDs 100 times per second and then automatically adjusts the light output to ensure the right colour point is achieved.
In another solid state lighting application, Nokia N-generation mobile phone uses a CCD camera as a sensor to detect the ambient light and automatically adjusts the WLED flash light levels. However, this is not an energy efficient method as the CCD is a powered light detector and consumes battery life.
US 2003/0222264A1 describes an LED acting as a photosensor fabricated using emitter materials InGaAlP, GaAsP and GaP. The photosensitivity is designed to match eye responsitivity. The device emits light in the narrow range of 590 to 630 nm whilst detecting light in the range of 500 to 620 nm.
U.S. Pat. No. 6,445,139 discloses a RGB LED luminaire with electrically adjusted colour balance. Here, analogous to other prior art mentioned above, a separate LED acts as a photosensor. By selectively turning off the electrical RGB LEDs' drive current the photodiode is intended to measure the light output for each colour LED separately. This information is then fed back to adjust the colour emitted by the luminaire.
Using a similar concept U.S. Pat. No. 6,664,744 describes a backlight where the backlight LED acts as an ambient light sensor. There is known a backlight which switches on automatically when the device in which it installed is moved (on the assumption that this indicates that the user wishes to use the device), but in such devices the backlight switches on regardless of the ambient light level. U.S. Pat. No. 6,664,744 teaches that the output from the ambient light sensor is used to prevent the backlight from switching on in bright ambient lighting conditions when the backlight is not necessary, to reduce power consumption. U.S. Pat. No. 6,664,744 also teaches adjusting the brightness level of the backlight according to ambient lighting conditions. FIG. 8 describes this invention where a backlight LED 101 and a resistor 150 are connected in series to the output and input pins of a microprocessor 151. The amount of photocurrent generated by the LED is measured by the microprocessor, which accordingly adjusts the forward driving conditions of the LED. This circuit has limited applications and can only be used in passive backlight devices (such as a backlight in remote controls or key pad backlights in mobile phones) and cannot be extended to controlling the LCD backlights where the amount of ambient light and colour variation detected by the backlight LED will be governed by the image that the LCD is displaying per frame.
WO2006/012737 discloses an illumination system in which the light-emitting elements may be switched between a first emission mode and a second detection mode. When a light-emitting element is in detection mode, it is able to detect light emitted by other light-emitting elements which has a wavelength equal to or shorter than the emission wavelength of the light-emitting element in detection mode. A second red light-emitting element is provided to act as a detector to measure light emitted by the red light-emitting element, but this further red light-emitting element is driven only in detection mode and does not contribute to the output of the illumination system.
JP 2006-260927 relates to a similar system, in which the LEDs of the system may be driven either in emission mode or in detection mode. Two red LEDs, two green LEDs and two blue LEDs are provided, so that one red LED may be used as a photodetector to detect emission from the other red LED.